Random copolyester compositions derived from various reactive mixtures of alkylene diols, aromatic dicarboxylic acids and/or their reactive equivalents and aliphatic dicarboxylic acids and/or their reactive equivalents have attained a high degree of commercial success for use in a wide variety of applications involving the bonding together of various materials. Depending on the precise application involved and/or the nature of the materials to be bonded together these copolyesters are generally employed either in the form of a hot melt or as a solution adhesive wherein the copolyester is dissolved in a suitable solvent. Furthermore, in many instances, such copolyesters must generally be tailored to the specific application for which they are intended if the proper balance of mechanical, chemical and thermal properties required by the application is to be achieved.
Although, as noted above, random copolyesters have found widespread useage in bonding applications, one area in which random copolyesters have not found ready acceptance is in the area of bonding agents for adhering metallic oxide pigments to magnetic tape substrates. Copolyesters which have been investigated for use as solution adhesives in this particular product application either did not have the proper balance of mechanical, chemical and thermal properties, or if they did, they contained low molecular weight cyclic oligomeric contaminants, believed to be formed during the preparation of the copolyester, which migrated to the surface of the magnetic tape during storage. Subsequently these contaminants would be wiped off on electric tape recording and pickup heads resulting in serious audio, visual and/or other interference problems. As a result, the most predominant elastomeric materials in use in the magnetic tape industry today for bonding metallic oxide pigments to magnetic tape substrates are the thermoplastic polyurethanes.
In contrast to the copolyesters previously investigated, applicant has now discovered a class of linear random copolyesters which possess the proper balance of mechanical, chemical and thermal properties, form stable solutions in common organic solvents at high concentrations and which are completely or essentially free of any low molecular weight cyclic oligomeric contaminant. This combination of properties and characteristics makes the copolyesters of this application excellent agents for bonding metallic oxide pigments to magnetic tape substrates.